17mm is pretty wide on a full frame camera - do you really want to go to 12mm? the distortion would be pretty overpowering for most subjects.
I don't know much anout the sigma lens but I can say the 17-40mm is excellent.

You need to think about FoV, not millimeters. The FoV difference between a 17mm and a 24mm is huge. The FoV difference between a 50mm and a 57mm is marginal. The FoV difference between a 100mm and a 107mm is almost invisible.

The 17-40 will be wide enough on your 5D. 17mm in FF terms is ultra wide.

I would recommend the 17-40L. I have owned both the lenses you are considering. The Sigma 12-24 while a good, sharp lens is quite a bit bigger than the Canon and has a very vulnerable bulbous front element. It is very difficult to make filters work with it especially on full frame. In the end I used big square filters held in front of the lens - not ideal. The Canon 17-40L is also a good, sharp lens. It is smaller and much more manageable for filters such as the polariser or UV.

As far as the 12mm to 17mm range goes I think you would find the perspective at the ultrawide end is rather gimmicky and wild. The overlap of the focal lengths will not go to waste entirely as it will save you having to chnage lenses so often - a consideration with the rather dust-spot prone 5D. (Don't get me wrong the 5D is a superb camera)

One more plug for the 17-40L, and also note that due to the overlap of some of the FL range using this lens with your 24-105, will lead to you needing far fewer lens swaps than if you were to go with the 12-24. You'll come to appreciate this...

You want to the 17-40. I never bother to mount the 12-24 on the 5D. It just never happens. These days I tend to got out with the 5D 17-24 and a A710IS in my pocket for longer shots (I am not much of a long shooter).

Save your money and get the 16-35L. It's just another $600 over the 17-40L. If you can afford a 5D you can wait a little longer and do it right. Slow lenses are a waste on such a fine DSLR. Better to get the F2.8.

Ah, but "only" another $600 is a personal assessment.. hold for you no doubt, but I don't feel there's an "only" there... $600 is a lot of money. And the F4 will do almost everything the 2.8 does. One stop is not that big a deal!

Do save your money and get Canon's highest quality UWA zoom for your 5D (the aforementioned 16-35). You'll be VERY happy you did; you owe yourself *at least* one fast lens (Canon lingo: "fast means f/2.8 or wider) for your 5D. There are no downsides to that lens and no serious competition either.

You need just one filter, a thin circular polarizer; if you feel paranoid then also get a UV filter for front element protection.

Do save your money and get Canon's highest quality UWA zoom for your 5D (the aforementioned 16-35). You'll be VERY happy you did; you owe yourself *at least* one fast lens (Canon lingo: "fast means f/2.8 or wider) for your 5D. There are no downsides to that lens and no serious competition either
Well, yes, a fast lens is good - I have the 24-70 2.8L, a very nice all-round lens. But I use the 17-40 4L and the 70-200 4L a lot more. So is it worth $600 more to have the wide 2.8? I';d personally put that towards the 24-70. Your mileage may vary - just what works for me.

I love my 16-35 so much, I refer to it as 'Mr." It's super and worth every penny if you need
the speed. However, digital allows ISO change on the fly and maybe the 17-40, a a lower
cost, is all you'll ever need. I would not look at any other lenses for a wide zoom need.

that 17mm shot of "1666" is neat. What and where (GB?) is that facility, or is that a manipulated photo? It almost looks like a doll house set. I love my 17mm, wish it was po$$ible to make a shift 17mm

first, this has been discussed at length on multiple forums. But, to oblige you, I'm going to have to be honest and disagree with the majority of posters here. The 17-40 is not a High Quality UWA. It suffers from some serious problems. Distortion, vignetting, corner sharpness, loss of contrast in the corners. and all of these are noticeable on the 5d.

I'd suggest the 16-35 as I've heard it is quite good, but ultimately even that, canon's flagship UWA is inferior when compared to zeiss, olympus, and nikon wide primes. If you need zooms and AF, by all means get the 17-40 or save for the 16-35. but ultimately I opted for a 21mm and 18mm zeiss and olympus combo. each of which could be found for roughly 700 including adapter. And both clearly beat any canon wide offerings.

"do you really want to go to 12mm? the distortion would be pretty overpowering for most subjects."

it pays to be precise here. In terms of the traditional definition of distortion - rendering parallel lines as parallel - the 17-40 has somewhat bad distortion, and the Sigma is the best performing UWA zoom available. The Sigma 12-24 is sometimes referred to as "the architectural photographer's friend" for that reason alone, completely beside the fact that it goes to 12mm FF for interiors.

I suspect what you meant instead is "perspective convergence," where vertical lines lean inward as you tilt the camera upward. It can be difficult to use a 12mm FF field of view, though no doubt all of us have seen architectural interior photos done skillfully with this lens and never even realized it. Successful outdoors and landscape photography with 12mm FF is quite a bit more rare. I'd wager that an event photographer could put it to good use though, given enough light.

However I'd agree that for a general purpose all-arounder for a FF camera, the 12-24 is a bit unique and specialized and the 17-40 is pretty great.

The wonderful thing about having a full-frame DSLR is that you get your wide angles back that those of us in 1.6 land sorely miss sometimes.

I second the suggestion that you save your cash for the 16-35 2.8L. The extra stop is worth it, and the sharpness of this lens is superb, probably amongst the best I've seen on any Canon lens. I hire one occasionally for my 30D, and the results are phenomenal. I would buy one, but I do a lot of low-light shooting, and the lack of a stabilizer made me decide in another direction (EF-S 17-55 2.8), but since it is EF-S, it doesn't apply to your situation.

Actually, what I'd suggest while you're waiting is to find a local rental shop and hire the lens when you specifically need it, until you are able to buy it.

On a camera such as the 5D, I wouldn't really consider third-party lenses. The quality of the files this camera generates can only be properly exploited by genuine Canon glass.

I agree with the other posters. I made the same decision a half year ago and bought the 17-40, and it is by far my favorite lens to use. However, I would trade anything to make it a 16-35. If it's at all possible, do it. you're going to want to upgrade it sooner or later. f/4 just sucks.

"On a camera such as the 5D, I wouldn't really consider third-party lenses. The quality of
the files this camera generates can only be properly exploited by genuine Canon glass."

hahaha... really.. i did not know that..
no offense, but seriously,.. how can you discount some of the best wide primes ever
made.. it's very ignorant... look around, there are comparisons, and tests all over the
place.. unless you're completely set on zooms and af.. seriously research and think about
third party alternatives that are cheaper, and in a lot of cases better.. that's right.. i said
better.. than canon's offerings.. it's not a sin guys.. it's the truth.

http://www.16-9.net/lens_tests/18mm_testb.html (mind you this is against the 16-35..
canon's best wide zoom)

Having owned the Sigma 12-24 I found it to be a pretty good lens. Build quality is very good. It is big and heavy, but probably no more inconvenient to carry around than a 75-300 zoom. The bulbous front element looks somewhat vulnerable, but in practice I never found it to be a problem at all. It is actually below the metal rim/fixed hood so is quite well protected.

The HSM worked well, and is about as good as canon's USM.

You can forget pretty much using filters with this lens. 85mm filters can be screwed into the two part lens cap if needed but you will get vignetting at 12 mm.

Colour and contrast were fairly good, sharpness was ok stopped down but a bit below other lenses I have used. While it stands up to 8x10 prints they won't be exceptionally sharp. I used it on a 300D and on a film body. However, in practice it was almost impossible to come up with a good composition at 12 mm on a FF body.

If I were looking for an ultrawide on the 5D, I think the 17-40 would be my first choice. 17 mm is plenty wide on FF.

>> Save your money and get the 16-35L. It's just another $600 over the 17-40L. If you can afford a 5D you can wait a little longer and do it right. Slow lenses are a waste on such a fine DSLR. Better to get the F2.8.

1. The OP specifically said "My budget will not stretch to 16-35L".

2. No lens test or a bulk of user reviews convinced me that the 16-35/2.8 is optically superior to the 17-40/4. On the contrary. The more I read, see and hear I am more and more convinced that their basic optical quality (sample variation aside) is the same.

3. I tried both and decided to go for 17-40/4. Reasons: I found them to be optically similar (surprise.....) with the 17-40/4 having an edge! in flare resistance. In a sunny place like Israel that is enough for me to go for the more flare resistant lens. I repeat. Even if they were priced the same I'd go for 17-40/4 for that reason alone. The fact that they are not made it a no-brainer decision.

4. When mounted on FF camera both lenses behave as they were designed for: UWA lenses. They do not behave like normal zooms as they do when they are mounted on 1.6X cameras. Therefore, and in contrast to normal zooms and tele lenses, I see little need to go for a faster aperture in UWA lenses.

A sincere thank you to all who responded. I found the comments very helpful.

For those suggesting the 16-35 I'm afraid I've already gone way beyond what the treasurer is comfortable with by opting for the 5d. One thing I've learned is that when you're on thin ice the last thing you do is start jumping up and down. There is no wiggle room in this budget.

Given that I want the best lens I can afford now it's looking like the 17-40. Although I have taken the advice given on board and have both lenses on order at the local shop and will have a good look at them before making my final decision.

One final point - what filters do people put in front of these lenses? Here I'm taking about gel filters such as Lee / Cokin. Are these systems wide enough for a 17mm UWA?

Well, the 17-40 is an excellent 2nd choice to the 16-35. No doubt you'll love the 17-40. Canon's high precision focus points demand f/2.8 or wider but if you're outdoors it's not as big a deal nor the dimmer viewfinder from an f/4 lens (especially on a FF 5D).

Again, a low profile circular polarizer is great aid and tool. Have fun!

I don't know what you're using the lens for, but if it's mostly for near-far everything in focus compositions, you wouldn't be using auto-focus - you'd be better off using hyper-focal method. In this case an f/2.8 lens with better AF wizbang is worthless, you can do with the 17-40L or as mentioned, sharper non canon lens with EOS adapters. If you're using the wide end mostly (12-24mm), you will have a very narrow DOF...again making an f/2.8 nearly worthless, I guess the viewfinder would be brighter...but if you're having problems seeing through with f/4 then maybe you're better off spending the $600 towards an eye doctor.